1. Field of the Invention
This invention relates to a method of manufacturing methanol.
2. Description of the Related Art
Jpn. Pat. Appln. KOKAI Publication No. 1-180841 discloses a method of manufacturing methanol (CH2OH) from hydrocarbon such as natural gas. Specifically, this publication describes a method of manufacturing methanol, which comprises:
reacting a gaseous hydrocarbon or a vaporized liquid hydrocarbon, by making use of a reformer, with water vapor in the presence of a nickel catalyst at a temperature ranging from 800 to 1000° C. to produce a synthesis gas comprising, as main components, hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2);
allowing reaction of the synthesis gas to take place over a copper-based methanol-synthesizing catalyst which is placed inside a synthesis reactor at a pressure of 50 to 150 atm and at a temperature of 200 to 300° C. to produce crude methanol;
cooling the crude methanol;
separating the cooled crude methanol into unreacted gas and liquid crude methanol; and
distilling the liquid crude methanol in one or more distillation columns to separate refined methanol and a waste water containing organic compounds having a lower boiling point than that of methanol (hereinafter, referred to as low boiling point organic compounds), another organic compounds having a higher boiling point than that of organic acid and methanol (hereinafter, referred to as high boiling point organic compounds).
In the aforementioned synthesis gas-producing step, by making use of a carbon dioxide recovery apparatus, carbon dioxide is recovered from a combustion exhaust gas which has been discharged from the reformer, and the carbon dioxide thus recovered is fed to the upstream side of the reformer and/or the downstream side of the reformer to obtain a synthesis gas having a desired molar ratio of H2/(CO+CO2) which is suitable for the manufacture of methanol.
Further, in synthesizing the crude methanol, the crude methanol is separated into liquid crude methanol and unreacted gas containing rich hydrogen by making use of a gas-liquid separator. A predetermined portion of this unreacted gas is recycled to the upstream side of the synthesizing reactor, while a redundant portion of this unreacted gas is entirely returned to the combustion device of the reformer to enable this redundant portion of unreacted gas to be utilized as part of fuel.
According to the conventional method of manufacturing methanol, hydrogen-rich purge gas is entirely utilized as a fuel for the reformer. However, when all of the hydrogen-rich purge gas is combusted as a fuel for the reformer, the quantity of water vapor in the combustion exhaust gas to be generated in the combustion device of the reformer is increased as compared with the case where a natural gas containing hydrocarbons as main components is combusted as a fuel in the reformer. If a combustion exhaust gas containing such a large quantity of water vapor is to be cooled by introducing it into the cooling column of the carbon dioxide-recovering device, a large quantity of cooling heat quantity is required.